On the other hand, would it really have been that interesting to delve into their pasts and backgrounds? They were/are heartless mass-murderers for hire, most readers probably would have seen that at worst as a feeble attempt to make excuses for the inexcusable, at best as irrelevant...

I think the Marauders are awful characters. But I wouldn't necessarily mind reading the origin stories of other heartless murderers for hire if done well.